To control the braking system of an aircraft, it is already known to use a pressure controlled servo-valve to deliver fluid under pressure to pistons which operate brakes that are disposed on the wheels of the aircraft undercarriage. In rough outline, such braking systems include means for varying the pressure applied to brake actuators such as pistons or jacks as a function of an electrical control signal which is derived from various electrical signals, including a signal resentative of a command from the pilot as detected by suitable sensor means.
Said means are generally known, and the present Applicant has already filed patents for such systems.
However, and again very much in outline, said signals are usually generated as a function of the position of a control pedal operable by the aircraft pilot. The position of the pedal is converted into an electric signal which is applied to the input of a servo-valve. Such a device is described, in particular, in the Applicant's published French patent applicaton No. 2 212 498.
It is recalled that such a servo-valve is controlled by means of electric currents which are applied to magnetic windings to set up a magnetic flux tending to change the position of fluid control means which in turn vary the pressure obtained at a utilisation output from said servo-valve.
In some such devices, the delivered pressure is at a maximum when the input current is nil, and conversely the pressure is nil when the input current is at a maximum.
In any event, generally speaking, the result is that the fluid pressure at the utilisation output from such a servo-valve is proportional to the extent to which the brake pedal is depressed.
In this way, when a pilot wishes to apply the brakes of the aircraft, he depresses the brake pedal to a greater or lesser extent in the same way as do the drivers of cars. However, in the present case, the transmission means from the pedal to the brake is not entirely hydraulic, but is partially electric and partially hydraulic. It is only on reaching the servo-valve that the electrical signal is transformed into a fluid pressure signal for controlling the brake pistons, and in particular the pistons on disk brakes located near the undercarriage wheels.
The braking system summarised above is relatively simple, at least in theory. It therefore gives good results and is also practical and cheap.
However, under some circumstances it has a drawback. The accuracy of the braking effect obtained depends mostly on the characteristics of the servo-valve. ie. on the quality of the transformation of the electrical signal at its input into a useable fluid pressure at its output.
Thus on some servo-valves, differences have been observed between the input signals (in electrical form) and the output signals (in fluid form) due, for example, to manufacturing tolerences. However, such differences can also be due to the servo-valve ageing, which ageing may be aggravated by the use of hydraulic liquids that are corrosive.
Preferred embodiments of the present invention provide a servo-valve system for controlling a vehicle brake actuator, eg. for an aircraft, in such a manner that vehicle braking is independent of variations in the characteristics of servo-valves used, and in which the values of said characteristics are monitored so that an appropriate alarm signal is be generated, if necessary, thereby increasing the safety of a vehicle braking system.